1. Field of the Invention
The present invention relates generally to a disc cartridge for a disc-shaped record medium such as a magneto-optical disc or the like in which signals can be recorded and/or reproduced while the disc is accommodated within the disc cartridge. More particularly, this invention relates to such a disc cartridge having a disc cartridge body or housing of substantially the same mechanical strength over the whole periphery thereof.
2. Description of the Prior Art
A disc cartridge which accommodates a disc-shaped record medium such as a magneto-optical disc or the like is known in the prior art. In such previously-proposed disc cartridge, an opening is formed in a flat portion of a cartridge half to expose part of a recorded region of a record medium. When the opening of the disc cartridge is opposed to a record,,reproduce head of a recording and/or reproducing apparatus, the record medium can be recorded and/or reproduced while the record medium remains within the disc cartridge. In such prior-art disc cartridge, the cartridge body is large and thick overall in order to provide sufficient mechanical strength.
When the prior-art disc cartridge as loaded onto a recording and/or reproducing apparatus, due to its thickness, the distance between the record medium accommodated in the cartridge housing and a recording and/or reproducing system of the recording and/or reproducing apparatus is increased considerably. In particular, when the record medium is a magneto-optical disc, an external magnetic field generally is applied to the magneto-optical disc at a side opposite to that irradiated by light beams from an optical head. If a coil or permanent magnet which is provided as an external magnetic field generating means for generating the above-described magnetic field and the disc surface have a large distance therebetween, then it is necessary to supply the external magnetic field generating coil with a very large current, which creates problems such as large power consumption, heat generated by the coil and so on. Further, a large size permanent magnet must be used as the external magnetic field generating means, which results in a large size recording and/or reproducing apparatus and a large size mechanism for inverting the magnetic field.
In order to solve the aforementioned problems, it has been proposed that the upper and lower cartridge halves of the cartridge housing or body be reduced in thickness in order to reduce the distance between the magneto-optical disc, the optical head and the external magnetic field generating means. However, this proposal creates unavoidable problems. That is, if the upper and lower cartridge halves are reduced in thickness, then the cartridge body is unavoidably warped and deformed, which causes the record medium to be deformed by the stress produced by the deformation of the disc cartridge body. As a result, the ability of the disc cartridge to protect the record medium, which is one of the most important features of the disc cartridge, is reduced, and the reliability of the disc cartridge is lowered.